Preheat oven to 350 F. Lightly butter a bundt pan.In a large bowl, whisk together flour, sugar, baking powder and baking soda.In a small saucepan, combine butter, cocoa powder, salt and 1 cup water over medium heat until butter is melted and warm, about 2-3 minutes. Pour mixture over the dry ingredients and stir using a rubber spatula until just combined. Beat in eggs, one at a time. Add sour cream and vanilla and beat until smooth. Spread batter evenly into the bundt pan. Bake for 20-25 minutes, until a tester inserted in the center comes out clean.Remove and let cool for 15 minutes before turning it onto a wire rack.To make the glaze, pour heavy cream and sugar in a small saucepan over medium heat. Stir until sugar is dissolved. Remove from the heat and stir in chocolate and corn syrup. Whisk until smooth. Let it sit for a couple of minutes befor you drizzle the glaze over the top of the cake, allowing it to drip down the sides.Allow glaze to set a little before serving.